The ability to separate a particle/fluid mixture into its separate components is desirable in many applications. Physical size exclusion filters can be used for this purpose, where the particles are trapped on the filter and the fluid flows through the filter. Examples of physical filters include those that operate by tangential flow filtration, depth flow filtration, hollow fiber filtration, and centrifugation. However, physical filters can be complicated to work with. As the filter fills up, filtration capacity is reduced. Also, using such filters requires periodic stopping to remove the filter and obtain or clear the particles trapped thereon.
Acoustophoresis is the separation of particles and secondary fluids from a primary or host fluid using high intensity acoustic standing waves, and without the use of membranes or physical size exclusion filters. It has been known that high intensity standing waves of sound can exert forces on particles in a fluid when there is a differential in both density and/or compressibility, otherwise known as the acoustic contrast factor. The pressure profile in a standing wave contains areas of local minimum pressure amplitudes at its nodes and local maxima at its anti-nodes. Depending on the density and compressibility of the particles, they will be trapped at the nodes or anti-nodes of the standing wave. The higher the frequency of the standing wave, the smaller the particles that can be trapped due the pressure of the standing wave.
The use of a single planar acoustic standing wave has been used to accomplish this separation process. However, this single, planar wave tends to trap the particles or secondary fluid in a manner such that they can only be separated from the primary fluid by turning off the planar standing wave. This does not allow for continuous operation. Also, the amount of power that is needed to generate the acoustic planar standing wave tends to heat the primary fluid through waste energy.
Conventional acoustophoresis devices have thus had limited efficacy due to several factors including heat generation, limits on fluid flow, and the inability to capture different types of materials. Improved acoustophoresis devices using three dimensional acoustic standing waves combined with improved fluid dynamics which allow the acoustophoresis particle separation process to be a continuous process driven by gravity are presented herein.